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Monitoring of 137Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived 137Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983–2010), the activity concentrations of 137Cs, mainly from fallout, were decreasing exponentially. Effective 137Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) 137Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, 137Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of 137Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 1014 Bq, meaning that 0.44 × 1014 Bq of FDNPP-derived 137Cs was added to the estimated global fallout 137Cs inventory in 2016 (0.53 × 1014 Bq). The net increase of 137Cs inventory in the Japan Sea through the addition of FDNPP-derived 137Cs accounts for approximately 0.2% of the total 137Cs flux from the plant to the ocean from the accident. [ABSTRACT FROM AUTHOR]

We have evaluated the contribution of sources of 137Cs to the inventory of radiocesium in waters (surface area: 6160 km2, water volume: 753 km3) off Fukushima Prefecture and neighboring prefectures from May 2011 to February 2015. A time-series of the inventory of 137Cs in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to February 2015 (1.89 TBq). The 137Cs inventory about four years after the accident was approximately twice the background inventory of 1.1 TBq. The magnitudes of the 137Cs influxes from sources into offshore waters for periods of 182-183 days were estimated from the first period (1 October 2011 to 31 March 2012: 15.3 TBq) to the last period (1 October 2014 to 31 March 2015: 0.41 TBq). We assumed that three sources contributed 137Cs: continuous direct discharge from the Fukushima Dai-ichi Nuclear Power Station (FNPS) even after the massive discharge in late March 2011, desorption/dissolution from sediments, and fluvial input. Quantification of these sources indicated that the direct discharge from the FNPS is the principal source of 137Cs to maintain the relatively high inventory in the offshore area. [ABSTRACT FROM AUTHOR]